Cancers are a leading cause of death in animals and humans. The exact cause of cancer is not known. There is evidence that certain activities such as smoking or exposure to carcinogens may enhance the risk for certain types of cancers and tumors.
Treatment of cancer in the early stages typically comprise local treatment such as, surgery and/or radiotherapy. While radiation therapy has been widely used in managing cancerous diseases, it is limited by lack or radiosensitivity of specific regions of malignant tumors. More advanced disease is treated by combining local treatment with chemotherapy. Although current chemotherapeutic agents have been shown to be effective against cancers and tumor cells, the use of combined treatment with all three regimens, surgery, radiotherapy, and chemotherapy, have not been shown to be effective against all cancer and tumor cells.
Much of the effort in therapeutics of cancer has focused on cancers that are metastasized. To date hormones, in particular estrogen, progesterone and testosterone, and some antibiotics produced by a variety of microbes, alkylating agents, and anti-metabolites form the bulk of therapies available to oncologists. Prostate cancer treatments, for example, rely on hormonal manipulation. However, in despite the initial high response rate, patients often develop hormone-refractory tumors. Unfortunately, the clinical usefulness of these treatments have been limited. This is because these therapies demonstrate only marginal levels of activity or generally unacceptable levels of cytotoxicity or both, thereby diminishing their usefulness in cancer treatment. Overall the results of cytotoxic chemotherapy have been disappointing indicating a long felt need for a new approach or treatment. Ideally cytotoxic agents that have specificity for cancer and tumor cells while not affecting normal cells would be extremely desirable. Unfortunately, none have been found and instead agents which target especially rapidly dividing cells (both tumor and normal) have been used.